MOSCOW -- Russian investigators say a prominent Ukrainian separatist has been shot dead in an upscale restaurant outside Moscow by a gunman disguised in a fake moustache and a panama hat.

Investigators said the most likely motive in the September 19 slaying of 40-year-old Yevhen Zhylin, who is sought by authorities in his native Ukraine on terrorism charges, is a business dispute, although they added that they were exploring other possible motives that include personal enmity.

Authorities said the gunman waited 20 minutes at a neighboring table for Zhylin, who arrived with another man for a business meeting. After they placed their order, the suspect rose, approached them, and opened fire.

Zhylin reportedly died on the scene, while the second man, identified by local media as Andrei Kozyrev, is hospitalized in critical condition.

A former police officer from the eastern Ukrainian city of Kharkiv, Zhylin was a key pro-Russian figure opposing the Euromaidan unrest that unseated President Viktor Yanukovych and led to a more Western-oriented government in Kyiv.

Zhylin led a militant anti-Euromaidan group called Oplot (Stronghold), which evolved out of a local fight club of the same name. The organization sent fighters, known as “titushki,” to Kyiv to combat and menace antigovernment protesters. Oplot enjoyed an ambiguous relationship with the authorities in Kharkiv.

After Yanukovych fled Ukraine in February 2014, Ukraine’s new authorities opened a criminal investigation into Zhylin, but he had fled the country. An Oplot offshoot was soon set up in Donetsk and reportedly helped seize strategic buildings from Kyiv's control in eastern Ukraine in April 2014, contributing to the Moscow-backed conflict that has now claimed over 9,600 lives.

In March 2015, Ukraine’s security service accused Zhylin of funneling money from Kharkiv firms to two separatist groups -- known as the Luhansk People’s Republic (LNR) and the Donetsk People’s Republic (DNR) -- that control territory in eastern Ukraine. In November, a Ukrainian military prosecutor wrote that Zhylin was being tried in absentia for financing “terrorism.” He was reportedly arrested in absentia in February.

Zhylin’s Oplot mixed-martial-arts club says it is currently based in Moscow. On September 20, its website posted a message of “sincere condolences” to Zhylin’s family, calling him a “real man with a kind heart and an open soul.”

Aleksandr Borodai, a former self-proclaimed leader of the DNR, told Kommersant on September 20 that he believed the motive for the murder could be a business dispute. Borodai also said Zhylin could have been killed for his activities in Ukraine, implying an assassination carried out by a Ukrainian nationalist or security operative.

“The more obvious version is that the [Russian] Federal Security Services are defending the space from those who could give evidence about the initial stage of Russia's hybrid war against Ukraine," Tymchuk speculated. "Zhylin and his Oplot were one of the main elements used by the Russian secret services to destabilize [Ukraine]. In other words, the Kremlin is removing the participants in those events.”

Unmarked Russian forces occupied Crimea before its annexation from Ukraine in a move that Russian President Vladimir Putin acknowledged only many months later. Moscow has consistently denied its forces are active in the separatist fighting in eastern Ukraine despite considerable evidence to the contrary.